Several minor edits, removed exact value of arXiv's size-limit

This commit makes several copyediting changes to the appendices and to the
supplement.tex introduction to the appendices.

The ArXiv unofficially increased upload limit of 50 Mb comes from a tweet:
https://nitter.fdn.fr/arxiv/status/1286381643893268483 (archive:
https://archive.today/PdxhT) but not listed on official ArXiv pages. So it
seems safer not to quote a value. The very old value was 0.5 Mb - out of
respect to people with low bandwidth, especially scientists in poor
countries. Tweets are generally not acceptable as "reliable sources" in
en.Wikipedia.

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